1. Field of the Invention
The present invention relates generally to a weak signal and anti-jamming Global Positioning System (GPS) receiver and method, and in particular, to a weak signal and anti-jamming Global Positioning System (GPS) receiver and method using full correlation grid detection and estimation.
2. Description of the Relation Art
Global Positioning Systems (GPS) are widely utilized in both commercial and military applications. Whether determining the location of a cellular telephone having a GPS tracking systems, navigating a commercial airliner or military jet, or guiding ordinance to particular targets, the reliance on GPS to provide accurate positioning is increasing.
Along with the increased reliance upon the GPS comes a growing need to provide more accurate and reliable GPS positioning. GPS signals can be adversely affected by various factors including, for example, low signal strength, noise and GPS jamming devices. Each of these causes the GPS system to provide false and inaccurate positioning data in the particular applications. In turn, this inaccurate or false data can cause devastating effects.
Many systems have been devised to counteract the adverse effects of the various factors. One such attempt is to increase the coherent integration time used to compute the input to frequency discriminators in a frequency locked loop. This method requires message bit prediction to work at high jamming levels (i.e.  greater than 42 dB). Operational constraints often make full message bit prediction impracticable.
One system that attempts to increase jamming immunity by optimizing processing gain for GPS/INS (Inertial Navigation System) systems is disclosed in U.S. Pat. No. 5,983,160 issued to Horslund et al. on Nov. 9, 1999. The Horslund et al. system includes a signal processor for receiving and down-converting GPS signals to a baseband frequency to provide I and Q (sine and cosine) signals. An integrate and dump operation is performed on the I and Q signals to provide GPS range and range rate residuals which are uncorrelated from sample to sample. A Kalman filter estimates navigation state corrections from the GPS range and range rate residuals. A generic navigation (NAV) function, responsive to the output of the Kalman filter and range error and velocity error signals, provides guidance information and signals for correcting position, velocity, and IMU (Inertial Measurement Unit) alignment states which are dynamically calculated from the range and range rate measurements. A line of sight geometry function maps position and velocity vector information from the NAV function into geometric range and range rate scalar information and feeds the scalar information to the signal processor and circuitry to perform the integrate and dump operation. This system improves the performance of the frequency locked loop by increasing the coherent integration time prior to the nonlinear frequency discriminator.
It is, therefore, an aspect of the present invention to provide a weak signal and anti-jamming Global Positioning System (GPS) receiver and method, and in particular, a weak signal and anti-jamming Global Positioning System (GPS) receiver and method using full correlation grid detection and estimation.
In accordance with another aspect of the present invention, there is provided a weak signal and anti-jamming Global Positioning System (GPS) receiver and method, and in particular, a weak signal and anti-jamming Global Positioning System (GPS) receiver and method using full correlation grid detection and estimation for use in military and spaceborne applications that require inertial navigation computations and inertial measurements.
The foregoing aspects of the present invention are realized by a system for increasing the accuracy of a Global Positioning System (GPS) receiver having an antenna for receiving a plurality of GPS signals from a plurality of GPS satellites, a down converter for down-converting the plurality of received signals, and an analog-to-digital (A/D) converter for converting the plurality of down-converted signals to a plurality of digital signals, each signal having an I (in) and Q (quadrature) phase component, comprising a correlator array for correlating each of the plurality of digital signals by multiplying the I and Q phases by carrier and code plus a correlation-grid location offset, accumulating the results of the correlation, and latching the results until a predetermined time; a coherent combiner for coherent combining the accumulated results at the predetermined time; a non-coherent combiner for non-coherent combining the coherent combined results for a time period sufficient to raise a correlation peak to a predetermined level above a noise floor to produce a plurality of non-coherent bin observables; a correlation grid detector for determining a maximum value of the plurality of bin observables and comparing the maximum value to a predetermined threshold, determining grid locations of bin observables having maximum values greater than the predetermined threshold, determining a bi-quadratic surface using the grid locations, and determining an argument maximum of the bi-quadratic surface; a correlation grid estimator for estimating epoch pseudorange and Doppler variables based on the argument maximum; and a Kalman filter for estimating errors in position, velocity, orientation, user clock, frequency, and ionosphere states on the estimated epoch pseudorange and Doppler variables.
The system can further comprise an orbit propagator for determining position and velocity by correcting for the estimated errors, or an inertial navigation computation module and an inertial measurement unit for determining position, velocity and orientation by correcting for the estimated errors.
In addition, a method for increasing the accuracy of a Global Positioning System (GPS) receiver having an antenna for receiving a plurality of GPS signals from a plurality of GPS satellites, a down converter for down-converting the plurality of received signals, and an analog-to-digital (A/D) converter for converting the plurality of down-converted signals to a plurality of digital signals, each signal having an I (in) and Q (quadrature) phase component, is also disclosed, comprising the steps of correlating each of the plurality of digital signals by multiplying the I and Q phases by a carrier wave and code, adding a location offset, accumulating the results of the correlation, and latching the results until a predetermined time; coherent combining the accumulated results at the predetermined time; non-coherent combining the coherent combined results for a time period sufficient to raise a correlation peak to a predetermined point above a noise floor to produce a plurality of non-coherent bin observables; determining a maximum value of the plurality of bin observables and comparing the maximum value to predetermined threshold; determining grid locations of bin observables having maximum values greater than the predetermined threshold; determining a bi-quadratic surface using the grid locations; determining an argument maximum of the bi-quadratic surface; estimating epoch pseudorange and Doppler variables based on the argument maximum; estimating errors in at least one of position, velocity, orientation, user clock, frequency, and ionosphere states on the estimated epoch pseudorange and Doppler variables; and determining at least one of position, velocity and orientation by correcting for the estimated errors.